1. Field of the Invention
The present invention relates to an image forming apparatus and a control method thereof and, more specifically, to an image forming apparatus that forms images by electrophotography and a control method thereof.
2. Description of the Related Art
With an image forming apparatus such as copier, printer, and MFP (Multi-Functional Peripheral), previously, the electrophotography has been widely popular. With the electrophotography, a laser beam or others are directed to a photoconductive drum for forming an electrostatic latent image thereon, and the resulting electrostatic latent image is developed using a toner.
For an image forming apparatus of electrophotography, the tandem system has been well known for color printing. Such a tandem image forming apparatus generally includes four photoconductive drums respectively corresponding to four colors of yellow (Y), magenta (M), cyan (C), and black (K). The photoconductive drums each form a toner image of its own color in parallel, and the resulting toner images of four colors are transferred to a paper for overlay one on the other so that a full-color image is formed. These four images are processed almost at once in parallel, and thus printing of a full-color image can be completed at high speed.
With electrophotography, generally, the photoconductive drums are each formed with an electrostatic latent image on its surface by a laser beam scanning the photoconductive drums in the main scanning direction. The laser beam is the one coming from a laser beam source such as laser diode. For scanning in the main scanning direction as such, a rotating multi-faceted reflective member called polygon mirror is often used.
As described above, the tandem image forming apparatus includes four photoconductive drums respectively corresponding to four colors of Y, M, C, and K. Such a previous tandem image forming apparatus is of a general configuration including four laser beam sources and four polygon mirrors respectively corresponding to the four colors. As a result, the size of hardware is large compared with an image forming apparatus specifically for monochrome printing.
In consideration thereof, there is a technology for apparatus downsizing by putting a laser beam source and a polygon mirror in use for sharing, i.e., one laser beam source and one polygon mirror (an example includes US 2007/0279723 A1).
US 2007/0279723 A1 describes the technology for forming four different inclination angles (inclination angles with respect to the rotation axis) to a reflection surface of a polygon mirror disposed in a rotation direction thereof. With such a reflection surface formed with the different inclination angles, a laser beam entering from any one laser beam source is reflected in directions varying with colors in a direction range orthogonal to the main scanning direction (direction of an elevation angle), thereby directing the laser beam toward the four photoconductive drums disposed at each different position. With the technology described in US 2007/0279723 A1, an optical lens (e.g., f-θ lens) is also put in use for sharing for placement between the polygon mirror and each of the photoconductive drums, thereby being able to reduce the hardware size to a considerable degree.
The issue here is that, even if a polygon mirror is put in shared use as described above, the optical path varies before reaching the respective photoconductive drums from the polygon mirror. As a result, the optical paths from the polygon mirror to the respective photoconductive drums are not always the same in length.
The range of a scanning angle for scanning by the rotation of the polygon mirror in the main scanning direction is the same no matter which color. However, if the optical paths from the polygon mirror to the respective photoconductive drums vary in length, images on the photoconductive drums will vary in magnification (image size) in the main scanning direction depending on which color. This thus causes a problem of out of color registration or others when images of four colors are overlaid one on the other.